"At the present range front we have demonstrated that the toads are locally adapted through this physiological challenge," says Jessop.

In their march across arid Australia, cane toads face the twin challenges of extreme heat and low humidity.

"Outside of the wet season, that environment is basically bone dry for eight months of the year, and the toads have to face extraordinarily high temperatures," says Jessop.

On the one hand the toads must lose water to cool down through evaporation - the equivalent of our sweating - so they don't overheat.

On the other hand, they need to conserve water so they don't dehydrate.

Cane toads at the invasion front need to optimise this trade-off to survive the extended arid zone dry season.

"When you are caught between a rock and hard place all you can do is try and sit in the middle," says Jessop.

Stress hormones

The toad's water balance is influenced by stress hormones, says Jessop, which help organisms adapt their physiological and behavioural responses to the environment.

As it gets hotter, stress hormones increase the amount of water loss for cooling purposes, but this also increases the risk of death from dehydration.

So the toad must have just the right amount of stress hormone: too little and will suffer heat exhaustion; too much and it will become dehydrated.

Jessop and colleagues wanted to find out if toads were successfully using the stress hormone to adapt to arid environments, or whether they were physiologically stressed and unlikely to advance any further.

"Can they keep going or have they hit the wall?"

Tanami Desert study

In a study of around 300 toads in the Tanami Desert, the researchers injected some individuals with a chemical that increased their stress hormone level, while the others were left untreated.

Jessop and colleagues measured the effect of stress hormones on the rate of water loss and survival in both groups to confirm the role of stress hormones in controlling water loss.

Importantly, they found that the survival rate in the untreated toads was higher than those manipulated to have higher stress hormone levels.

This told the researchers that the toads were adapting and still had the potential to continue their invasion come the next wet season.